One of society's biggest challenges as we enter the new milleunium is the elimination of industrial pollution. This high level of consumption continues despite new international treaties that severely limit the amount of volatile organic compounds (VOC) that can be released in plant effluents. The abbreviation VOC has become synonymous with a plethora of social, economic, and ecological hazards. It is thus imperative that the research community explores alternatives to VOCs and develops alternative processes that reduce the health risks, safety concerns, and environmental issues associated with current industrial solvents.
One of the largest segments of the chemical industry is the polymer industry, with over 30 million tons of polymers produced annually. A major component of industrial waste in this industry is used solvent, and strategies for eliminating solvent vapors will have huge importance in cleaning up industrial production of polymers. While aqueous reaction media are widely used (emulsion and suspension polymerization), not all polymerizations are amenable to these conditions, and solution polymerization using VOCs is still widely practical. At the workshop on The Role of Polymer Research in Green Chemistry and Engineering, sponsored by the National Environmental Technology Institute and held at the University of Massachusetts on Jun. 11-12, 1998, the use of supercritical fluids and RTILs (Room Temperature Ionic Liquids) as alternative media for polymerization was proposed. To our knowledge, only the former alternative has been systematically explored.
Recently, DeSimone and co-workers (J. L. Kendall et al.; D. A. Canelas et al.; K. A. Shaffer et al.) have developed the use of supercritical CO2 as an alternative medium for polymerization. Since most industrially important hydrocarbon-based polymers are relatively insoluble in CO2, heterogeneous polymerization techniques are often required. Dispersion polymerizations, characterized by soluble monomers, initiators, and surfactants but insoluble sterically-stabilized polymer products, have been successfully conducted in supercritical CO2 for methylmethacrylate (Y.-L. Hsiao et al.; J. M. DeSimone et al.; Y.-L. Hsiao et al.; K. A. Shaffer et al.; M. L. O'Neill et al.; M. L. O'Neill et al.; C. Lepilleur et al.; T-M Yong et al.), styrene (D. A. Canelas et al.; D. A. Canelas et al.; H. Shiho et al.), divinylbenzene (A. I. Cooper et al.), acrylonitrile (H. Shiho et al.), and vinyl acetate (D. A. Canelas et al.; J. S. Shih; J. S. Shih et al.; C. Bunyakan et al.). A key to successful dispersion polymerization in CO2 has been selection/synthesis of suitable polymeric stabilizers (usually copolymers) (J. L. Kendall et al.; D. A. Canelas et al.; K. A. Shaffer et al.). It should be noted that precipitation polymerization, which is identical to dispersion polymerization except the particles are not stabilized, is also a widely practiced technique even for water soluble polymers (J. S. Shih; J. S. Shih et al.; C. Bunyakan et al.).
The free radical polymerization process is a key process for the polymer synthesis industry. Robust and economical, it accounts for about 50% of all mass-produced polymers. (C. H. Bamford) One of the major virtues of radical polymerization is that it is relatively insensitive to monomer and solvent impurities, thus these polymerizations can be carried out under relatively undemanding conditions as compared to ionic polymerizations. Another advantage of this process is its ability to be applied to a wide range of monomers. The study of free radical polymerizations has been boosted recently by the development of variety of “living” techniques. (C. J. Hawker; J. S. Wang et al.; M. Sawamoto et al.)
Methods for synthesizing and using block copolymers is also of great interest, academically and industrially. This is particularly due to their spontaneous self-assembly into well-ordered nano-domains, which is a result of the combined effects of mutual repulsions between incompatible chain segments and the constraints imposed by the connectivity of the blocks. (H. Narita et al.) Many methods are available to synthesize well-defined block copolymers, including anionic, cationic and controlled (“living”) radical polymerization. However, these polymerizations are done in volatile organic solvent media, which has been blamed for the increasing air pollution.
Over the past several years, RTILs have stimulated much interest among chemistry community for their potential as green “designer solvents” and several excellent reviews are available recently. (Y. Chauvin et al.; P. Wasserscheid et al.; T. Welton; J. D. Holbrey et al.; R. L. Hussey; R. T. Carlin et al.) RTILs not only show potential for use in separations and as electrolytes, but they are also promising solvents for chemical syntheses and particularly for catalysis, including their applications in polymerizations.
In the early 1990s, several polymerization studies were done in chloroaluminate based ionic liquids. Carlin and Wilkes (R. T. Carlin et al.; R. T. Carlin et al.) in 1990 observed polymerization of ethylene during two studies of titanium chemistry in chloroaluminate (III) ionic liquids to which alkylchloroaluminate (III) drying agents had been added. Carlin and coworkers (R. T. Carlin et al.) investigated the electrochemistry of TiCl4 in AlCl3-1-ethyl-3-methylimidazolium chloride (ImCl) melt and they found the combination of TiC4 and AlEtCl2 in AlCl3-ImCl to be catalytic active for ethylene polymerization. Even though the yield of polyethylene was very low, they demonstrated that an RTIL could serve as polymerization medium. Using Cp2TiCl2 instead of TiCl4, higher yields of polyethylene were achieved as reported by Carlin and Osteryoung. (R. T. Carlin et al.) The electrochemical polymerization of benzene in various ionic liquids to prepare poly(p-phenylene) has also been reported. (D. C. Trivedi; V. M. Kobryanslii et al.; L. M. Goldenberg et al.; S. A. Arnautov; R. T. Carlin et al.) In these studies, ionic liquids were used mainly as convenient electrolytes. Carlin and Osteryoung (V. M. Kobryanskii et al.) produced a new electroactive material by electrochemical oxidation of triphenylsilyl chloride (Ph3SiCl) in acidic ionic liquid (AlCl3-ImCl). The film exhibits reversible redox behavior and is electronically conducting in oxidized state. They postulated that the cations of the ionic liquids took part in the formation of the film. However, one of the major drawbacks of chloroaluminate(III) based ionic liquids is that they are water-sensitive.
An alternate technique, “living” radical polymerization, offers great control over molecular architecture, molecular weight, and molecular weight distribution (C. J. Hawker; M. Sawamoto et al.; T. E. Patten et al.). Nitroxide-mediated polymerizations, using additives like TEMPO (2,2,6,6-tetramethylpiperidinoxy), are generally sluggish and require long polymerization times in bulk monomer at elevated temperatures (ca. 125° C.) for 2-3 days (M. K. Georges et al.). Transition metal (Ru (II) or Ni (II)) catalyzed and Cu (I) catalyzed atom transfer radical polymerizations generally proceed at lower temperatures (M. Sawamoto et al.; T. E. Patten et al.) and are usually run in VOCs. By use of additives such as camphorsulfonic acid (M. K. Georges et al.) or acylating agents (C. J. Hawker), the kinetics of TEMPO systems can be greatly accelerated. Thus all of these initiator systems can now be employed in solution (M. Sawamoto et al.). C. Kafetzopoulos et al. reported the synthesis of block terpolymers in such a “heterogeneous living radical polymerization”. In heterogeneous radical polymerization of certain monomers, the lifetimes of the propagating radicals can be extended from milliseconds (for homogeneous systems) to a few hours by trapping them in insoluble polymer particles (C. Kafetzopoulos et al.; T. Sato et al.). Block terpolymers of N-methyl methacrylamide (NMeMA) with styrene and isoprene were synthesized by heterogeneous radical polymerization of NMeMA followed by addition of the other monomers (C. Kafetzopoulos et al.). The existence of living macroradicals was confirmed by electron paramagnetic resonance spectroscopy (EPR); the formation of block copolymers was confirmed by differential scanning calorimetry (DSC) and infrared (IR) spectroscopy (C. Kafetzopoulos et al.). Recently, Carmichael et al. used 1-butyl-3-methylimidazolium hexafluorophosphate ([Bmim]PF6, also known as [C4mim]PF6), an air and water-stable RTIL, as a solvent for the Cu(I)-N-propyl-2-pyridylmethanimine mediated “living” radical polymerization of methyl methacrylate (MA). They found that the rate of polymerization is enhanced as compared to other polar/coordinating solvents, even though the molecular weight of the resulting poly (methyl methacrylate) (PMMA) is low. Moreover, the polymer recovered is made essentially copper free by a simple solvent wash, which avoids the contamination of the polymer product by the catalyst.
Regarding copolymers, Guerrero et al. described the synthesis of polystyrene-b-poly(ethylene oxide) (PS-b-PEO) using a macroinitiator process. The PS and PEO blocks were linked via bridges containing bezopinacole groups. Since then several other studies have appeared in the literature. (H. Narita et al.; B. Hazer et al.; S. Nagarajan et al.; R. B. Seymour et al.) Narita and coworkers synthesized block copolymers of styrene and methyl methacrylate (MMA) (PS-b-PMMA) by using a polystyrene (PS) macroinitiator obtained through oxidation of imino functionalized PS. (H. Narita et al.) This functional PS was prepared by using 2,2′-azobis[(2-imidazolin-2-yl)propane] dihydrochloride as the initiator for styrene polymerization. Hazer and coworkers obtained PS-b-PMMA block copolymers through a polymeric peroxycarbamate compound. (B. Hazer et al.) Nagarajan and Srinivasan obtained poly(acrylonitrile)-b-poly(ethylene glycol)-b-poly(acrylonitrile) (PAN-b-PEG-b-PAN) block copolymers from a ceric ion redox process. (S. Nagarajan et al.) All these macroinitiator processes require post-polymerization reactions to activate the “protected” radical initiator. Seymour et al. reported high levels of block copolymer formation when macroradicals were produced in viscous poor solvents, e.g. silicone oils. (R. B. Seymour et al.; R. B. Seymour et al.; Stahl, G. A. et al.) To minimize the production of free radicals that would produce homopolymer of the second monomer, they kept the system at 50° C. for 96 hours prior to addition of the second monomer (2,2′-azobis(isobutyronitrile) (AIBN) was used as initiator). More recently, Kafetzopoulos et al. prepared block terpolymers of N-methyl methacrylamide (NMeMA) with styrene and isoprene (PNMeMA-b-PS-b-PI and PNMeMA-b-PI-b-PS) by heterogeneous radical polymerization in benzene. P. E. J. Louis et al. have raised doubts about the trapped radical mechanism, suggesting that residual primary radicals could result in a mixture of graft and statistical copolymers being formed. Noda and Watanabe have described the formation of polymer-in-salt electrodes, formed by in-situ polymerization of vinyl-monomers in ILs.
Thus, while many polymerization strategies have been explored, there is an ongoing need for environmentally responsible methods of polymerization.